1. Field of the Invention
The present invention relates to an inverter and inverter driving method, and more particularly, to an inverter and inverter driving method for enabling soft start of a load to prevent improper operation of the inverter.
2. Discussion of the Related Art
An inverter is a voltage supply apparatus converting DC voltage into AC voltage. An inverter is broadly used for household appliances including washing machines as well as display devices such as a monitor and a TV. Particularly, an inverter transformer is widely used as a high voltage generator for turning on and off a discharging tube broadly used for inverter stands, fluorescent stabilizer, monitor, notebook computer, and so on.
An inverter driving circuit is necessary since an inverter is driven through an outside control method such as Pulse Width Modulation (PWM).
Hereinafter, an inverter of related art is explained referring to the accompanying drawings.
FIG. 1 is a circuit diagram illustrating an inverter connected to a load, such as a Cold-Cathode Fluorescent Lamp (CCFL) for a TFT-LCD.
As shown in FIG. 1, an inverter of the related art comprises an inverter driving circuit 10 and a transformer 20. The inverter driving circuit 10 comprises a inverter control unit 11, a transistor Q1, and a resistor R1. The transformer 20 converts DC voltage supplied after an electric source is turned on by a controller (not shown) into AC voltage to be provided for a lamp 30. The lamp 30 operates by receiving the AC voltage supplied by the transformer 20. Lamp current is provided to the inverter control unit 11 as a feedback. The inverter control unit 11 controls the transistor Q1 according to the level of the feedback lamp current from the lamp 30, thereby controlling the output voltage of the transformer 20.
Hereinafter, operation of the inverter driving circuit 10 is explained.
First, the inverter control unit 11 supplies a pulse signal having a predetermined duty ratio to the gate of the transistor Q1. Brightness of the lamp 30 may be controlled by changing the duty ratio of the pulse signal. When the transistor Q1 is turned on by the pulse signal supplied to the gate of the transistor Q1, DC voltage B+ is inputted to the transformer 20 with a certain interval. The transformer 20 converts the DC voltage B+ into AC voltage according the number of turns in its primary and secondary windings. The AC voltage outputted from the transformer 20 is provided to the inverter control unit 11 as a feedback. Subsequently, the inverter control unit 11 varies the pulse width of the pulse signal to be supplied to the gate of the transistor Q1, thereby controlling the output voltage of the transformer 20 within a predetermined range.
Referring to FIG. 2, when the inverter is turned on, a kick-off period is needed to start the lamp, during which time maximum lamp voltage and lamp current should be supplied to the lamp. For this, the inverter control unit 11 varies the frequency of the pulse signal by changing the resistance of the resistor R1 so that the high output voltage of the transformer 20 may be provided to the lamp 30 during the kick-off period. Typically, when 600-700 V is needed for usual operation of the lamp 30, 1600-1700 V is needed during the kick-off period.
In summary, during the kick-off period, high output voltage from the transformer 20 is supplied to the lamp 30 for starting the lamp 30, and then the feedback lamp current from the lamp 30 is provided to the inverter control unit 11. Based on the feedback lamp current from the lamp 30, the inverter control unit 11 controls the resistor R1 and the transistor Q1 to provide the lamp 30 with an appropriate level of voltage and current.
However, the inverter of the related art has problems as follows.
First, since the output voltage from the transformer 20 is considerably high during the kick-off period, the lamp current level is also raised. Due to the raised lamp current level, the output voltage of the transformer 20 may be raised to deviate from the predetermined range. Thus, the lamp 30 may be turned off when it should be on.
Secondly, as it is required to provide a considerably high level of voltage to the lamp 30 during the kick-off period, the DC input voltage B+ of the transformer 20 may have a level lower than that required for the high level of AC output voltage to be supplied to the lamp 30. This may also cause improper operation of the inverter and unwanted turning off of the lamp 30.